Between 1990 and 2001, engineers worked to stabilize the Leaning Tower of Pisa by removing approximately 70 tonnes of soil from under its northern side, which was the higher side. This under-excavation technique straightened the tower by 44 centimeters. The tower had been leaning at 5.5 degrees, which exceeded the calculated tipping point of 5.44 degrees. The method preserved the authenticity of the monument by avoiding the use of concrete and permanent modern materials. The soft soil, which caused the tilt, paradoxically cushioned earthquakes over eight centuries. Closure to the public was decided in January 1990, following a little more than 31 million visitors in sixty years. This decision was influenced by the collapse of the civic tower of Pavia a year earlier, which caused deaths. The tower's tilt was reduced to 3.97 degrees, the same as it had at the beginning of the 19th century. The tower reopened to visitors on December 15, 2001, after eleven years of closure. The defect in the tower's structure goes back to its construction, which began in 1173 on soft clay soil unable to support such a mass without uneven yielding. Medieval builders attempted to compensate by adding stone on one side, which is where the slight banana shape described by engineer Roberto Cela comes from. Cela leads the Opera Primaziale Pisana, the organization responsible for the monuments in the square. The first measurements involved surrounding the tower with steel, cabled and anchored to the ground, then placing lead counterweights on the northern side (a total of 870 tonnes). This slowed the movement without reversing it. The construction site, directed by engineer Michele Jamiolkowski, mobilized specialists from about fifteen countries. The main operation took place between 1998 and 2000, in small successive steps, under constant supervision. The monument they moved weighs double the structure of the Eiffel Tower, which approaches 7,300 tonnes. At the top, the tower regained 44 centimeters. Reinforcing the foundations with concrete or mechanical pushing would have imposed a shock on an eight-century-old building resting on clay. The team chose to work with the ground rather than against it, leaving no permanent modern material in the structure. The soft soil, which caused the problem, also cushioned earthquakes that could have brought down a more rigid structure. The construction site cost approximately 28 million euros according to estimates. Since 2001, the tower has continued to straighten itself slowly without any intervention, about 4.5 centimeters more between 2001 and the end of the 2010s, bringing the total gain to about 45 centimeters since 1990. Experts consider it stable for at least two centuries, but the engineer warns that it will never be perfectly straight due to the curvature inherited from its construction.